1. Field
The present disclosure generally relates to optical-modulator circuits. More specifically, the present disclosure relates to an optical-modulator circuit that includes three-dimensional waveguide tapers.
2. Related Art
Silicon photonics is a promising new technology that can potentially provide low-power, high-bandwidth and low-latency interconnects in future computing systems. However, in order to implement practical silicon photonic links, efficient light modulators are needed. Note that it is complicated to construct efficient light modulators because the electro-optic effect in silicon (Si) is weak. As a consequence, a number of different types of modulation mechanisms are being investigated. Two promising modulation mechanisms are the electro-absorption associated with the quantum-confined stark effect (QCSE) in SiGe/Ge quantum-well (QW) devices, and the electro-absorption associated with the Franz-Keldysh (FK) effect in tensile-strained germanium (Ge).
QCSE provides a strong electro-absorption mechanism, and has been used to make high-speed, low-power and compact opto-electronic devices using III-V materials. In practice, electro-absorption associated with the QCSE in a multiple QW structure that includes germanium QWs, which are separated by silicon-germanium barriers, can offer a much stronger electro-absorption effect than a depletion-based silicon light modulator. Consequently, silicon-germanium QCSE devices can provide broadband operation with low driver voltage. In addition, the same QCSE device can be used as either a light modulator or a photo detector.
Similarly, increased electro-absorption (relative to silicon) can also be achieved using the FK effect in Ge1-x Six (for example, using the enhanced FK effect in tensile strained, epitaxial germanium-on-silicon). Because the FK effect takes place on a sub-pico-second time scale, the speed of the electro-absorption mechanism based on the FK effect is only limited by the RC delay, and can be designed to achieve very high bandwidth. Moreover, the same FK-effect device can also be used as a photo detector with high responsivity and high bandwidth.
However, it is very challenging to integrate these light modulators with silicon-based optical waveguides, which makes it hard to use these light modulators. In particular, it is very challenging to fabricate electro-absorption light modulators with sub-micron on-chip silicon optical waveguides, because epitaxial growth is needed for the active material layers in the electro-absorption light modulators, such as the multilayer QW structures or the tensile-strained germanium layer. (For example, the active materials in an electro-absorption light modulator may be lattice mismatched with silicon by 5%, and may require selective area growth to integrate with silicon optical waveguides.) This epitaxial growth is in a direction normal to the substrate, while the silicon optical waveguides carrying the optical signals are normally in the plane of the substrate. Moreover, it is difficult to couple the light from a sub-micron silicon optical waveguide to the active material layers to modulate the light, and then to couple the modulated light back to a sub-micron output silicon optical waveguide with low optical loss.
Hence, what is needed is an optical-modulator circuit that does not suffer from the above-described problems.